In recent years, wearable devices have been developed progressively with the spread of Internet of Things (IoT). Representative examples thereof include a watch and glasses that can be connected with internet. Wearable devices that can always monitor physical conditions are also necessary in a medical field and a sports field, and are expected to be a growth field in the future.
In the medical field, wearable devices have been investigated to monitor organic conditions such as an electrocardiogram measurement, which detects heart beats by concentration change of ions released from skin linked to the heart beats. The electrocardiogram is measured by fitting a body with electrodes on which electro-conductive paste is applied, and this measurement is performed only once in a short period of time. On the other hand, the aim of development of the foregoing medical wearable device is to develop devices that monitor health conditions continuously for several weeks. Accordingly, bio-electrodes used for a medical wearable device have to keep the electric conductivity unchanged and not to cause skin allergies even when being used for a long time. In addition to these, it is desirable that the bio-electrode is light in weight and can be manufactured at low cost.
Medical wearable devices include a type in which the device is attached to a body and a type in which the device is incorporated into clothes. As the type in which the device is attached to a body, it has been proposed a bio-electrode using water soluble gel containing water and electrolyte, which are materials of the foregoing electro-conductive paste (Patent Literature 1). On the other hand, as the type in which the device is incorporated into clothes, it has been proposed a means to use cloth in which an electro-conductive polymer such as poly-3,4-ethylenedioxythiophene-polystyrenesulfonate (PEDOT-PSS) or silver paste is incorporated into the fibers for electrodes (Patent Literature 2).
When using the foregoing water soluble gel containing water and electrolyte, however, the electric conductivity is lost as the water is lost due to drying. On the other hand, some people can cause skin allergies by the use of metal with high ionization tendency such as copper. The use of an electro-conductive polymer such as PEDOT-PSS also has a higher risk of skin allergies due to the strong acidity of the electro-conductive polymer.
One of the role of bio-electrodes include conversion of concentration change of ions released from skin to electric signals. Accordingly, they have to have higher ionic conductivity. The bio-electrode of water-soluble gel electrolyte has higher ionic conductivity. On the other hand, the use of metal having higher electron conductivity such as silver or gold as a bio-electrode causes inferior electric conductance and higher resistance between the bio-electrode and skin. It has been investigated to use metal nanowire, carbon black, carbon nanotube, etc., which have excellent electron conductivity, as an electrode material (Patent Literatures 3, 4, and 5). These bio-electrodes, however, fails to exhibit high performance by the reason described above.
To improve the ionic conductivity of solid-state batteries, it has been investigated to combine ionic electrolyte and polyethylene glycol. The ionic conduction is brought by ions hopping on the polyethylene glycol chain.
It has started to use silicone for use such as medical tubes and so on since silicone is excellent in biocompatibility and repels water such as perspiration. However, it is difficult to use silicone for bio-electrodes since silicone is an insulating material.
Urethane may be usable for bio-electrodes since urethane is also excellent in biocompatibility, and the electric insulation property is not so high as that of silicone. Urethane, however, has higher hydrophilicity and is hydrolysable, thereby being unsuitable for uses that involve contact with skin for a long time.
In order to prevent the hydrolysis of polyurethane, polyurethane having a silicone main chain has been investigated (Patent Literature 6).
When the bio-electrode is away from skin, it becomes impossible to obtain information from the body. Just the change of contact area fluctuates the quantity of electricity to be conducted, thereby fluctuating the baseline of an electrocardiogram (electric signals). Accordingly, the bio-electrode have to be in contact with skin continually without changing the contact area in order to obtain stable electric signals from a body. For that purpose, the bio-electrode preferably has tackiness. It also needs stretchability and flexibility to cope with expansion and contraction as well as change of bending of skin.
Urethane is processible to a soft gel state after curing. Water-containing bio-electrodes based on urethane gel have been proposed for the bio-electrode use described above (Patent Literature 7).